It is well known to spray apply hydraulic cementitious slurries to metal structural members in order to provide a heat resistant coating thereon. U.S. Pat. Nos. 3,719,513 and 3,839,059 disclose gypsum-based formulations which contain, in addition to the gypsum binder, a lightweight inorganic aggregate such as vermiculite, a fibrous substance such as cellulose and an air entraining agent. Such slurries are generally prepared at ground level and are pumped to the point of application, where they are spray applied to the substrate. Often the point of application exceeds 20 or 30 stories where high rise construction is involved. Accordingly, pumpability of the slurries is an important criteria in their formulation and preparation. Thus, they must be able to hold the large quantity of water that renders them capable of being pumped easily and to great heights, yet they must retain a consistency sufficient to prevent segregation or settling of ingredients and permit adequate yield or coverage of the substrate at a given thickness. The coating mixes must adhere to the substrate both in the slurried state and in the dry state. Also, the mix must set without undue expansion or shrinkage which could result in the formation of cracks that would seriously deter from the insulative value of the dry coating.
U.S. Pat. No. 4,751,024, the disclosure of which is incorporated herein by reference, teaches sprayable cementitious compositions containing shredded polystyrene as a lightweight aggregate in fireproofing compositions. The cementitious binder is preferably gypsum, but may instead be Portland cement. Indeed, Portland cement is typically used as the binder in applications involving high traffic areas, since the cement tends to be stronger than gypsum.
However, one problem associated with Portland cement-based fireproofing compositions is the relatively poor hangability (the thickness of material that can be applied in a single application) thereof with respect to gypsum-based compositions, since water tends to segregate in Portland cement mixtures in view of the relatively slow hydration process. In addition, in view of the relatively long time it takes for Portland cement to set, only one spray application can be made per day on a given substrate. Where the thickness of that spray application is insufficient, a second application cannot be made until the first application sets. Premature attempts to coat a second application result in fall off. If multiple applications cannot be made in a typical eight hour work day, labor costs necessarily increase, as does the time in which a job can be completed.
Unlike gypsum-based compositions, which can be easily accelerated with alum, such as in close proximity to the spray nozzle as set forth in U.S. Pat. Nos. 4,904,503 and 4,934,596 (the disclosures of which are herein incorporated by reference), alum does not accelerate Portland cement in sufficient time to allow multiple passes in one day. In addition, sodium aluminate should not be used as an accelerator, in view of the corrosive nature of the sodium aluminate, which causes severe corrosion to the hot rolled steel substrates.
Accordingly, the fireproofing industry has long sought a Portland cement-based sprayable fireproofing material that can be applied to substrates such as steel in multiple applications per day, and that exhibits improved hangability notwithstanding such multiple applications.